Author

Date of Award

Document Type

Degree Name

Master of Science in Applied Physics (MS)

Administrative Home Department

Department of Physics

Advisor 1

Will Cantrell

Committee Member 1

Claudio Mazzoleni

Committee Member 2

Raymond Shaw

Abstract

Aerosol particles and cloud droplets are not static entities but are continuously interacting with their chemical environment and therefore changing in their properties [1]. Also, clouds are ubiquitously turbulent, so water vapor or other trace gases fluctuate in space and time. Aerosol properties such as number concentration, chemical composition and size influence many cloud properties including the clouds’ probability for precipitation and hence their lifetime. Also, an increase in the number of aerosol particles can lead to an increase in cloud droplet number and decrease the effective droplet radius, which results in an increase in a cloud’s albedo for a constant liquid water path. In response, clouds serve as a dominant removal mechanism for intermediate sized aerosol particles while other sizes are removed via other loss processes such as settling and diffusion. This feedback between aerosol and cloud is the process of cloud cleansing through which cloudy, polluted air from a continent is slowly transformed into cloudy, clean air of a maritime environment.

A facility, the Pi Chamber, so called because of its working volume of 3.14 m3 with the cylindrical insert in place is developed at Michigan Tech to study the interaction of gases, aerosols and clouds in a turbulent environment. Within the Pi Chamber, a turbulent environment is created via a temperature difference between the top and bottom surfaces, inducing turbulent Rayleigh B´enard convection. Cloud formation is xvii initiated by injecting NaCl aerosol from a constant output atomizer into the chamber. In our experiments, we employ different temperature gradients (hot on the bottom and cold on top) to drive turbulent convection. Once the cloud properties reach steady state,the aerosol source is turned off to initiate the cloud cleansing process. Size distributions of the interstitial aerosol are acquired continuously using a Scanning Mobility Particle Sizer (SMPS). Measurement of the distribution of interstitial aerosol as a function of time is used to derive characteristic decay times as a function of size. Our data suggest that for aerosol particles with dry diameters between about 40 nm and 100 nm, the principal removal mechanism is activation which proceeds in a two-stage process; ie decay of interstitial aerosol concentration occurs slowly at first, then more rapidly. Smaller particles of diameters about 10 nm to 20 nm are not activated but removed through convective diffusion. The cloud cleansing process thus demonstrates a simplified version of evolution of polluted clouds to a maritime state.